Silver and copper halide doped bi2te3-as2se3 thermoelectric material



United States Patent ,5 Claims. Cl. 252-623) My invention relates tothermoelectric materials and methods of their manufacture, andparticularly to materials of n-type electric conductance for use asthermocouple legs in Peltier cooling devices.

The two legs of thermocouples now preferentially employed for electriccooling on the Peltier principle consis-t of semiconductor materialshaving n-type and p-type conductance respectively. The suitability of asemiconductor material for use in such Peltier couples is characterizedby largest feasible thermoelectric etiectivity wherein at denotes thedifferential thermoforce, 0' the electrical conductance, and K thethermal conductance. It

' has been proposed to employ as n-type thermocouple legs asemiconductor material consisting of a mix crystal of bismuth telluride(Bi Te and arsenic selenide (As Se I have discovered that thethermoelectric properties, particularly the thermoelectric effectivityand hence the suitability for thermoelectric cooling, of suchmix-crystal semiconductors is greatly increased by providing a mixcrystal of the system Bi Te As Se for optimal n-type doping, with ahalide of metal from subgroup B in the first group of the periodicsystem (Cu, Ag, Au).

I have found that an especially effective, optimal degree of n-doping isobtained by giving such a mix-crystal semiconductor body a dopantcontent of 0.05 to 0.07% by weight of copper bromide or approximately0.1% by weight of silver iodide.

Particularly advantageous is the application of these dopant additionsin the semiconductor n-type mix crystal whose percentive molecularcomposition is in the range from 70 mole percent BizTeg and 30 molepercent As Se to 90 mole percent Bi Te and mole percent As Se Mixcrystals of the system Bi Te As Se doped for n-type conductance by meansof halogenides according to the invention and having, for example, amix-crystal composition of 80 mole percent Bi Te and 20 mole percent AsSe have been found to possess a thermoelectric etfectivity z of about4.O-10 degree This eifectivity is superior to all heretofore knownthermoelectric n-type materials at room temperature. The highest valuesof eifectivity heretofore known from n-type mix crystals of the systemBi Te Bi Se were up to 2.9-10 degree- I am not yet in a position toexplain the surprising discovery according to my invention that the mixcrystals, formed by the thermoelectrically not very effectivesemiconductors Bi Te and As Se if suitably doped and produced with theaid of the above-mentioned halogenides, particularly bromides andiodides of copper or silver, surpass in thermoelectric effectivity alln-type materials heretofore available for such purposes.

A mix-crystal material according to the invention, when properlycombined with any one of the known and suitable p-type thermocouplematerials known for such purposes, effects a more efficient Peltiercooling action resulting in a higher cooling power or a greaterreduction in temperature under otherwise comparable conditions. Sincethe invention does not concern itself with the particular p-type legs ofthe thermocouples nor with the particular design of the thermoelectricdevices, they are not further described herein. However, if desired,reference as to such information may be had to the copendingapplications Serial No. 212,411, filed July 25, 1962 now Patent No.3,211,656; Serial No. 195,441, filed May 17, 1962; Serial No. 150,701,filed Nov. 7, 1961; Serial No. 174,442, now Patent No. 3,220,199, filedFebruary 20, 1962; and Serial No. 192,254, filed May 3, 1962, assignedto the assignee of the present invention.

Suitable for the production of mix crystals according to the inventionare particularly a normal freezingmethod or the application of azone-meltingmethod, as will be apparent from the examples describedpresently.

EXAMPLE 1 (NORMAL-FREEZING METHOD) Stoichiometric quantities inpulverulent form of the four component-elements of the'mix-crystal Bi TeAs Se were employed with a purity degree of 99.999%. The powderquantities were placed into an elongated tubular quartz ampule togetherwith 0.05% by weight of copper bromide to serve as doping substance. Thequartz ampule was evacuated and then sealed. Thereafter, the quartzampule was placed into a melting furnace and heated to a temperature of600 C. After the ampule and its content had reached this temperature,the ampule was lowered out of the melting furnace at a rate of 0.6 cm.per hour so that the molten material was subjected to normal freezingprogressing from one end of the elongated ampule to the other. As aresult, a polycrystalline mix crystal of rod shape was obtained having adiameter of 10 mm., corresponding to the melting space in the ampule.

EXAMPLE 2 (ZONE-MELTING METHOD) The elements of the composition wereused with a purity of 99.999%. Added to the elemental components of themix crystal was about 0.1% by weight of silver iodide :toserve asdopant. The materials, employed in pulverulent form as in Example 1,were melted together in an evacuated quartz ampule of elongated shapeand thereafter permitted to solidify to a rod-shaped solid structure.Thereafter the rod in the quartz ampule, which remained evacuated andfused off, was subjected to zone levelling by pulling a melting zoneonce in forward and return direction lengthwise through the rod. Thezone temperature used was 600 C. The zone-pulling rate was 6 cm. perhour.

The following Table 1 indicates the thermoelectric properties of a miXcrystal according to the invention composed of mole percent Bi Te and 20mole percent As Se in dependence upon different amounts of copperbromide dopant. The indicated values relate to a temperature of 25 C. Itwill be noted from Table 1 that the optimal n-doping with copper bromideis in the range of about 0.05 to about 0.07% by weight of copperbromide.

The following Table 2 indicates the thermoelectric properties of threemix crystals of the system Bi Te As Se without dopant. The listed'values also relate to a tem perature of 25 C.

T able 2 [Three mix crystals of the system Bi Ie As Se at 25 C. withoutdopant Composition in mole percent It is apparent from Table 2 thattoward higher amounts of As Se a decreasing electric conductivity(increased specific electric resistance) is accompanied by a reductionin thermoelectric effectivity. Furthermore, the specimens becomeincreasingly vitreous with excessively high proportions of As Se Mixcrystals with a very high content of Bi Te obviously possess adisadvantageously high thermal conductance due to the influence of thestill hardly disturbed Bi Te crystal lattice. All of the mixcrystalalloys of the range covered by Table 2 can be improved with respect tothermoelectric efiectivity by having the above-mentioned halogenidedopants of the good-conductor metals from the subgroup B in the firstgroup of the periodic system of elements.

Also suitable as dopants in mix crystals of Bi Te and As Se for thepurposes of the invention are CuI, CuCl, CuF AgBr, AgCl, AgF, AuBr, AuI,AuCl. As a rule, copper halide can be used in a dopant quantity of about0.05% by weight of the mix crystal. The silver halide and gold halide,as a rule, can be used in a quantity of about 0.1 by weight.

I claim:

1. A thermoelectric semiconductor consisting essentially of an n-typemix crystal of Bi Te and As Se having a molecular composition between 70to 90 mole percent Bi '1e and 30 to mole percent As Se and containing asdonor dopant from 0.5 to 0.1% by weight a metal halide selected from thegroup consisting of the halides of silver and copper.

2. A thermoelectric semiconductor consisting essentially of an n-typemix crystal of Bi Te and As Se having a molecular composition between tomole percent Bi Te and 30 to 10 mole percent As se and containing 0.05to 0.07% by weight of copper bromide.

3. A thermoelectric semiconductor consisting essentially of an n-typemix crystal of Bi Te and As Se having a molecular composition between 70to 90 mole percent Bi Te and 30 to 10 mole percent As se and containingapproximately 0.1% silver iodide.

4. The method of producing a thermoelectric semiconductor consistingessentially of an n-type mix crystal of Bi Te and As Se having amolecular composition between 70 to 90 mole percent Bi Te and 30 to 10mole percent As Se and which comprises placing the mixcrystal componentsinto a quartz ampule together with halide of a metal selected from thegroup consisting of silver and copper of the periodic system in a dopantquantity from 0.05 to 0.1% by weight, sealing and evacuating the ampule,heating the ampule in a heating zone to a temperature of at least 600C., and gradually removing the ampule from the heating zone atnormalfreezing rate.

5. The method of producing a thermoelectric semiconductor consistingessentially of an n-type mix crystal of Bi Te and As Se having amolecular composition between 70 to 90 mole percent Bi Te and 30 to 10mole percent As Se and which comprises melting the mix crystalcomponents in an evacuated ampule together with halide of metal selectedfrom the group consisting of silver and copper in a dopant quantity from0.05 to 0.1% by weight, and subjecting the resulting body of material inthe ampule to zone levelling in a single forward and return pass of themelting zone at a zone temperature of about 600 C. and at a zone-pullingrate of about 6 cm. per hour.

References Cited by the Examiner Egli: Thermoelectricity, John Wiley &Sons, 1960, pp. l36-145.

TOBIAS E. LEVOW, Primary Examiner.

R. D. EDMONDS, Assistant Examiner.

1. A THERMOELECTRIC SEMICONDUCTOR CONSITING ESSENTIALLY OF AN N-TYPE MIXCRYSTAL OF BI2TE3 AND AS2SE3 HAVING A MOLECULAR COMPOSITION BETWEEN 70TO 90 MOL PERCENT BI2TE3 AND 30 TO 10 MOLE PERCENT AS2SE3, ANDCONTAINING AS DONOR DOPANT FROM 0.5 TO 0.1% BY WEIGHT A METAL HALIDESELECTED FROM THE GROUP CONSISTING OF THE HALIDES OF SILVER AND COPPER.